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    <script>
    (function(){
         var a=b=5;
    })();
    console.log(b);//5
    console.log(a);//报错
    // 这条语句实际上是以下声明的简写 b = 5; var a = b;，所以那个 b 相当于是window的属性，window.b = 5;
    </script>
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